1. Field of Invention
The present invention relates to a magnetic field detection apparatus and a rotation detection apparatus that includes the same. The present invention particularly relates to the configuration of the bias magnet of the magnetic field detection apparatus.
2. Description of the Related Art
An assembly that is made up of a magnetic field detection apparatus and a gear has been conventionally recognized as an example of rotation detection apparatuses for rotary devices that are used in the field of automobiles or the like. The magnetic field detection apparatus includes a magnetic sensor. The magnetic sensor generally has a magneto-resistive effect element that utilizes the magneto-resistive effect. A gear that is made of a magnetic material and that is connected to the rotary device is provided on the front side of the magnetic sensor. A bias magnet that applies a bias magnetic field to the magneto-resistive effect element is provided on the backside of the magnetic sensor. One of the two magnetic pole surfaces of the bias magnet (N pole surface) faces the magnetic sensor. The bias magnetic field is drawn toward a tooth of the rotating gear and the direction of the bias magnetic field is periodically changed. In other words, as the gear rotates, the component of the bias magnetic field that is parallel to the magnetic pole surface is periodically changed. The magnetic sensor detects the rotation of the gear by detecting the change of this magnetic field component.
The bias magnetic field is generally directed substantially perpendicular to the magnetic pole surface of the bias magnet at the center of the magnetic pole surface, but it is directed outward with respect to a surface perpendicular to the magnetic pole surface as it is farther away from the center. In other words, magnetic lines radiate outwardly from the magnetic pole surface. The component of the bias magnetic field that is parallel to the magnetic pole surface increases at a position away from the center of the magnetic pole surface. It is desirable that the bias magnetic field or the magnetic lines be directed as close as possible to a direction that is perpendicular to the magnetic pole surface of the bias magnet when no magnetic substances, such as a gear, are in the vicinity. In other words, it is desirable that the bias magnetic field have a small component in a direction that is parallel to the magnetic pole surface over a large area.
JP11-183498A and JP8-320327A disclose magnetic field detection apparatuses having two magnetic sensors. A recess is provided on a part of the magnetic pole surface, allowing the bias magnetic field on the front side of the two magnetic sensors to be directed in a direction perpendicular to the magnetic pole surface of a bias magnet. U.S. Pat. No. 8,080,993 discloses a magnetic field detection apparatus that includes a magnetic field guide element provided between a bias magnet and a magnetic sensor. The guide element has a V-shaped recess. The bias magnetic field is directed inward at both side slopes of the recess and is then turned outward. Therefore, the bias magnetic field is directed perpendicular to the magnetic pole surface of the bias magnet at a position where the magnetic field directed inward is turned outward.